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The gauche effect is very sensitive to solvent effects, due to the large difference in polarity between the two conformers.For example, 2,3-dinitro-2,3-dimethylbutane, which in the solid state exists only in the gauche conformation, prefers the gauche conformer in benzene solution by a ratio of 79:21, but in carbon tetrachloride, it prefers the anti conformer by a ratio of 58:42. [9]
The staggered conformation is more stable by 12.5 kJ/mol than the eclipsed conformation, which is the energy maximum for ethane. In the eclipsed conformation the torsional angle is minimised. staggered conformation left, eclipsed conformation right in Newman projection
Cis-1,4-Di-tert-butylcyclohexane has an axial tert-butyl group in the chair conformation and conversion to the twist-boat conformation places both groups in more favorable equatorial positions. As a result, the twist-boat conformation is more stable by 0.47 kJ/mol (0.11 kcal/mol) at 125 K (−148 °C) as measured by NMR spectroscopy. [9]
It was found that the most stable conformations had lower energies based on values of energy due to bond distances and bond angles. [6] In many cases, isomers of alkanes with branched chains have lower boiling points than those that are unbranched, which has been shown through experimentation with isomers of C 8 H 18. This is because of a ...
Thus, B is the most stable conformation. With certain polar substituents, hydrogen bonding can occur in the allylic system between the substituents. Rather than the strain that would normally occur in the close group proximity, the hydrogen bond stabilizes the conformation and makes it energetically much more favorable.
More complex molecules, such as butane, have more than one possible staggered conformation. The anti conformation of butane is approximately 0.9 kcal mol −1 (3.8 kJ mol −1) more stable than the gauche conformation. [1] Both of these staggered conformations are much more stable than the eclipsed conformations.
Another conformation of cyclohexane exists, known as boat conformation, but it interconverts to the slightly more stable chair formation. If cyclohexane is mono-substituted with a large substituent, then the substituent will most likely be found attached in an equatorial position, as this is the slightly more stable conformation.
A methyl substituent has a significantly smaller A-value than a tert-butyl substituent; therefore the most stable conformation has the tert-butyl in the equatorial position. The utility of A-values can be generalized for use outside of cyclohexane conformations. A-values can help predict the steric effect of a substituent. In general, the ...